NASA has been launching spacecraft into the final frontier for over 60 years, but getting rockets ready for lift-off is no easy feat. Rockets can take anywhere from 18 months to over five years, in the case of the now-retired space shuttle, to be constructed and cleared for lift-off. The process of making sure a spacecraft is safe for crew and payload alike is lengthy, precise, and fascinating. There are several stages that rockets go through before they get to the launch pad at Kennedy Space Center.
The Rocket and a Spare
When NASA constructs new spacecraft, it is standard procedure to make a duplicate. This isn’t a backup in case something goes wrong. In fact, the whole idea is for the twin rocket to go through a series of worst-case scenarios to see how it holds up.
Safety and stability tests take time, and the spaceflight industry is notorious for being behind schedule. Having a spare copy of the spacecraft allows engineers to conduct tests on one copy while the other is still under construction and development.
Sea Level Tests vs. Altitude Tests
The physical conditions on the ground are vastly different from what a rocket will face during launch. This goes beyond the absolute zero temperature out in space. The way gravity affects the rocket changes as it gets higher in the atmosphere and engineers need to account for that difference in their tests.
To accomplish this, some tests are classified as “sea level” tests while others are “altitude tests.” Sea level tests assess the performance of the rocket under the conditions at sea level, where it will be at lift-off. Altitude tests are a bit harder to simulate and require the use of a sealed chamber and sensitive equipment to mimic the conditions the spacecraft will experience in flight.
Vibration Tests
Vibration can be a big problem in spaceflight. Everything has what is called a natural resonant frequency. This is the frequency at which an object will naturally or automatically tend to vibrate. It is important for the engineers at NASA to study the spacecraft’s natural frequency, and see how it responds to intense outside vibration, because even a little disruption can be catastrophic.
The Pogo Effect
Liquid-fueled rockets, like NASA’s Apollo series, can experience something known as the pogo effect. This describes a disruptive vibration along the long axis, or vertical axis, of the rocket. It causes the engines to essentially get clogged and crammed with too much pressure from backed-up fuel. This leads to a “surge” of fuel from the engines, which changes the rocket’s velocity. Even a small deviation from the calculated velocity of the rocket at any point during lift-off is a big deal because all of the forces acting on the rocket are connected. So, a change in velocity changes the way that gravity is pushing up and down on the rocket.
The pogo effect gets its name because it can be so aggressive that the rocket literally appears to bounce up and down in flight. This puts excess gravitational pressure on the crew and/or payload of the rocket. If severe enough, it can leave astronauts unable to perform their duties during the flight.
Testing Structural Integrity
Engineers put spacecraft through rigorous vibration testing along each axis to make sure nothing like this happens, even if something were to go wrong during the flight. These tests use large motors to vibrate the spacecraft at various frequencies. Engineers put the rocket under intense pressure to make sure it isn’t going to rupture under strain; the safety of the crew inside is the top priority for all of the rocket’s safety tests.
The results are used twofold. First, the tests reveal the structural integrity of the spacecraft, showing if it holds up as it’s shaken around, whether at high intensity or low. Second, the data collected allows engineers to train the rocket’s flight computers.
As mentioned above, everything has a natural frequency. The rocket will vibrate at this frequency automatically whenever it is moved. The flight computers have to be able to differentiate between this natural frequency and vibrations caused by outside forces in order to be able to steer the rocket properly and give accurate data about its performance. Vibrations can reveal major underlying issues or threats to rockets, so it is important for engineers to get a clear understanding of how the rocket naturally vibrates before it can take to the skies.
Shock Testing
The shocks phase of the testing process includes everything from lightning to intended explosions on the ship. For example, when booster phases separate, it can jar the ship quite heavily. The rocket has to be able to continue performing optimally even in the midst of launching spent engine phases back to the earth’s surface.
Most American missions are launched from NASA’s Kennedy Space Center at Cape Canaveral, Florida. This location is ideal for launching rockets because it is close to the Earth’s equator, which actually rotates faster than the poles. This gives rockets a bit of a speed boost when blasting off. However, Florida weather includes high amounts of lightning, and the rocket has to be able to stand up to any shocks from this kind of inclement weather while preparing for launch on the pad.
Acoustic Tests
Along with physical vibrations, sound vibrations can also be a threat to spacecraft. Rockets get extremely loud when firing, to such an extent that they can damage other parts of the spacecraft as well as its payload (which often includes highly sensitive scientific instruments). This incredible noise intensity is part of why spectators at rocket launches from Cape Canaveral have to sit at least 2 miles away from the launchpad.
NASA’s acoustic testing process involves blasting the spacecraft with noise from upwards of 1,500 speakers. These intensive procedures need to be done in special facilities that can handle the massive amounts of sound while also maintaining the integrity of the test itself. Some tests are even done at night so results aren’t thrown off by noise or vibrations coming from activity around the facility. Acoustics testing can reveal structural weak spots on the spacecraft, including everything from poor welds to areas vulnerable to cracking.
Mock Countdowns
The final testing phase is a series of mock countdowns to simulate launch without the rocket actually leaving the launchpad. These tests are the last step to make sure everything on the rocket is functioning correctly. The real spacecraft that will actually complete the mission is used for these and it is loaded onto the pad a couple of weeks before the launch actually takes place (this is why it is so important for the rocket to be stable under poor weather conditions).
Aside from verifying the readiness of the spacecraft itself, mock countdowns also help the crew and mission control team practice for the launch. Everyone has to know exactly what to do at exact moments during the launch. With everything happening so fast during the real thing, there is no time for error or delays, and one forgetful moment could jeopardize the mission.
For NASA’s crewed missions, which have now returned to the U.S. thanks to private spaceflight company SpaceX, an additional step is added to the mock countdown testing phase. If a human crew is going to be aboard the rocket, they need to be able to jettison from it safely should anything go wrong during the launch. These abort systems need to be tested rigorously before NASA will clear the spacecraft to transport passengers.
Clear for Lift-Off
Once the spacecraft passes all these evaluations and tests with flying colors, it is officially ready for take-off. It is thanks to NASA’s intensive testing programs that rockets are safe for carrying American explorers to the stars. Not only do they make the spacecraft safer, but they also help engineers improve spaceflight technology and find new ways to advance American rocketry. So, with every long day of rocket exams, NASA’s engineers are getting us one step closer to the Moon and beyond!
Emily Newton is the Editor-in-Chief of Revolutionized, an online magazine discussing the latest technologies changing our world.